‘DREAM’: Thirty-five years after dressing as a Met for Halloween, David Einhorn is buying a stake in the Amazin’s. Photo:

‘DREAM’: Thirty-five years after dressing as a Met for Halloween, David Einhorn is buying a stake in the Amazin’s. (Getty Images)

As a 7-year-old, he posed for pictures in a homemade Mets jersey. Now, 35 years later, he’s going to buy a chunk of his beloved team.

Savvy stock picker and famed short seller David Einhorn, 42, yesterday called the chance to fork over $200 million for a minority stake in the money-losing Amazin’s “beyond my wildest childhood dreams.”

The hedge-fund guru — whose New York-based Greenlight Capital manages $7.8 billion in assets — has been a diehard Mets fan all his life.

“I spent my first seven years living in New Jersey and rooting for the Mets,” Einhorn said. “In 1975, I even dressed in a homemade jersey as a Met for Halloween.”

He lost track of that costume — he went as slugger Dave Kingman — after his family moved from Demarest, N.J., to Milwaukee.

There, the Brewers eventually won him over because one of his friends lived next door to then-owner Bud Selig, now baseball commissioner.

Einhorn recalled playing ball in his buddy’s back yard, noting, “If we hit the ball really, really far, it landed in the Seligs’ yard and it was a home run.”

As Major League Baseball boss, Selig must now sign off on any deal between Mets owner Fred Wilpon and Einhorn.

Einhorn now lives in a $6 million, seven-bedroom manse on three acres in Rye, Westchester County, with his journalist wife, Cheryl, and kids Rachel, 14, Naomi, 11, and Mitchell, 9.

As a boy, he wanted to be a ballplayer. He never made it past Little League — but was a standout on his high-school debate team.

“I have no athletic ability, so that’s a problem,” Einhorn once joked to DealBreaker, a business news and gossip website.

After graduating summa cum laude with a government degree from Cornell University in 1991, Einhorn went to work as a Wall Street analyst.

By 1996, he had launched Greenlight with just $900,000 — more than half of it from his parents, who run a Wisconsin mergers-and-acquisitions firm.

Einhorn made a killing betting against Allied Capital and Lehman Brothers with short positions of their stock.

Despite his business acumen, the youthful-looking big shot has boyish quirks. He takes afternoon naps and keeps a light saber, baseballs, teddy bears and toy cars in his office.

In 2008, Einhorn famously won $660,000 at the World Series of Poker — and gave it all to Michael J. Fox’s Parkinson’s research charity.

The Mets’ new suitor indulges his passion for baseball by coaching his kids’ Little League teams and playing in fantasy-baseball leagues with his son and Einhorn’s younger brother, Daniel.

Part of his fantasy-league strategy, Einhorn has said, is to be a “value buyer,” but he pounces if a top player comes up for cheap in the draft.

“I would close my eyes, pay up, break the budget and buy him,” he said. “It would be too good an opportunity to pass up.”

Einhorn said the Mets investment is long-term and that he’s not worried about the team’s current finances.

“Baseball is a tough sport,” he said. “There’s going to be losing seasons and tough seasons, winning seasons and, hopefully, championship seasons. I hope to experience all of those.”